Hoffman 2034 Question

[shortfuse]

Hey guys been away for a while too much work and not enough play.  But this job I am on now is finally coming to an end and I can start doing things I enjoy again.
I have a Hoffman 2034 i acuired about 2 years ago and it is actually built by Doug.  Great sounding amp.
It has recently started rejecting any pedal that gets plugged into it.  Including the tuner by it self.  It makes the sound that your amp usually makes when the input cable is plugged into your amp but not the guitar.  But if you run the cable from the guitar straight into the amp it is very quiet with the hot switch on and dead quiet with the hot switch off.  What would be the 1st plan of attack to start checking?  I am suspecting some thing in the preamp circuit I say this because the PI seems to be glowing quite a bit brighter than before and the other 2 per-amp tubes.  Any suggestions greatly appreciated.

[shortfuse]

Hey forgot to mention that I did try changing the pre amp tubes and it had no affect on it at all.  I will try to post voltages tonight.

[sluckey]

It has recently started rejecting any pedal that gets plugged into it.

Does this mean that a guitar plugged into the amp works fine, but if you use the same cable to connect any pedal into the same jack, it doesn't work? At all???

[shortfuse]

It has recently started rejecting any pedal that gets plugged into it.

Does this mean that a guitar plugged into the amp works fine, but if you use the same cable to connect any pedal into the same jack, it doesn't work? At all???

Sorry Steve I guess my post was a bit miss leading.  If you run through a pedal (any pedal which it never did before) to the amp it is very noisy and makes a sound an amp makes when it is unplugged from the guitar but on and still plugged in the amp.  It will play when you are running through the pedal it is just very noisy and touching the strings makes no impact on the noise level.  If you plug straight to the amp from the guitar it plays great and is almost dead quiet.  This amp does not have an effects loop.

[bobmegantz]

sounds to me like you have a bad cord...

[sluckey]

If you plug straight to the amp from the guitar it plays great and is almost dead quiet.

I would not blame the amp without first ruling out cables, pedals, pedal boards, cables, cables, etc. Sounds like an external problem.

[shortfuse]

I hear ya but it is not cables and I do not think I have 20+ bad pedals.  I have about 15 cables and I can take 2 that from the guitar to the amp alone do not make a noise but if you put a pedal (one at a time not a string) in between them and the amp it is very noisy.  And it does not matter which pedal I use, And some of these pedals have true bypass.  Bizzare as it sounds it happens and never did before.  Thats why i decided to ask here 1st before i started probing arround.  Maybe I will try to post a you tube vid and link so you can see what I am talking about.
I am starting to believe it could be Black forest genomes ????  Thank you for the replies.

[sluckey]

I'd look at the input jack, two resistors between the jack and first tube, and the first tube.

[Willabe]

I am starting to believe it could be Black forest genomes ???? 

Nope, no such thing.

Many times guys here and on other sites have had the same type of "unknown problem" and finally found it and fixed it.

You'll find it.

         Brad     

[HotBluePlates]

If you plug straight to the amp from the guitar it plays great and is almost dead quiet.

Then you have ruled out the guitar or amp as the problem.

First thing that came to mind for me was loose jacks in the pedals, weak batteries, bad cord, etc. But I'd agree with Sluckey, if anything in the amp is to blame, it is most likely just the input circuit.

None of these pedals are using a.c. cords or power supplies to power them, right? If so, malfunctioning supplies or ground loop hum comes to mind. Amp has a 3-prong cord with all three prongs still present, right? House doesn't have any wiring issues?

Don't laugh, I nearly went crazy trying to troubleshoot a guy's amp which made weird noises, but which I couldn't replicate when I got the amp to my house; his outlets had wiring faults. When I plugged it in at my place, all the gremlins went away.

[EL34]

Are you overdriving the input level?

If you have the hotswitch on, the pre amp signal level is already very high.

Turn off the hot switch and see what happens

[shortfuse]

I did not have the time to look at it today like I wanted to, I had to go in to work most of the day.

Steve
The 68K and 1m resistors are fine.  They are on the board and not the jack as in the wiring diagram (this amp is a bit different on the board layout and components than the schematic in the library).  The contact on the tip to lift the ground appears to be working correctly also.  I pretty much ruled out the 1st tube as I tried several with the same results.  But the hot switch ground is wired in through the input jack ground as in the hot switch diagram in the library.  I am going to follow that path further tomorrow.

Doug
It does it on or off.  It does it with modulation or even a tuner not just overdrive which is the part that baffles me.  Although at my experience level it does not take a whole lot to baffle me.  

HBP
Cords outlets are the same as the ones I have the Diezel and Fender plugged into no issues with those amps.  Yes all 3 prongs functional.  As for the pedals I have tried with dc adapters and batteries no differences.  I hear ya on the outlets I re capped a friends Twin and when I was done I got this snapping sound.  I chased it for 2 days and gave it back to him and told him he may want to send it in to a repair shop.  He took it home said no issues and sounded great.  Plugged my own amp in 2 days later same snapping sound bad outlet.

It is probably something blatantly obvious that I am over looking.  This may or may not be related but I am going to take voltages tomorrow as V3 is much brighter than V1 & V2.  

[EL34]

This may or may not be related but I am going to take voltages tomorrow as V3 is much brighter than V1 & V2.

That's just the filament string. Some are exposed more than others

You can record your own voltage chart here
http://www.el34world.com/charts/valve/ValveData.php

I have already recorded the voltages of one of my 2034 heads here
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10045

[shortfuse]

OK so I am all set up to take voltages got the 4 nuts off all 4 corners of the board carefully lift the board and lean it forward towards the knobs grab the meter and get to looking at the board 2 wires off the board wiper of the master and ground of the volume.  Now they were attached before I lifted the board but I was very careful in what I did so I don't think it was all me.  I decide to reattach the wires before proceeding and test the amp.  Its fixed no more noise even through the pedal board.  Maybe the previous owner before me messed around inside and cracked the wires where they land on the board and all it took was me to lift and move the board forward one more time?  I don't care the amp is working fine again.
I have added some gut shots of this amps board section since I had it open.  Doug I love the bias control in this amp where you can adjust left right or center both at the same time.  It allows you to use not fully matched sets how come you don't have these in your store???  I played around with a few sets of tubes tonight and I find the 6L6's sound the best to me. 
Once again thanks to all who helped steer me in the right direction.

[EL34]

You don't have to lift the board up to take voltage measurements.
The lugs on top of the board is were you read everything.

Not sure what you mean by the bias pots.
I have a small 3 legged pot in my store that you can use for bias
If you wanted to do what I did there, you need 3 of them

The 2034 diagrams are on the library page.

[shortfuse]

Ok the Black Forest Gnomes are back again.  I used the amp twice since the last post with no issues and 3 nights ago when I went to use it, again any pedal that was front ended into it, it does not like, and is noisy like a bad ground again.  And again if I go straight from the guitar to the amp it is very quiet not dead quiet but 90% better than going through a pedal.  I did try new pre-amp tubes or I should say I pulled a set from another amp with no issues and it did the same thing and I installed the ones from the 2034 in the other amp and everything was fine.  I did just receive my new input jack from Doug today and I am going to replace it tonight and see what happens.  The hot switch is grounded to the input jack and I may try moving it to see if that could be causing the issue.  68K & 1meg resistors prior to V1 are fine.

Doug
I did get the voltages with out lifting the board once I lifted it and saw what the bottom side looked like I saw what you mean and I was able to get all voltages from the top side.
I see that V2 has the same voltages as the one you posted except on the opposite side of the tube.  I wouldn't think that would be an issue?

Here are my voltages                               http://www.el34world.com/charts/valve/ValveData.php?e=view&f=13751

Here are yours from 12/18/2010                 http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10045

I have all the other voltages as on the layout schematic  http://www.el34world.com/Hoffman/images/2034skem.gif  in the library on a sheet I forgot to scan earlier so I will have to upload that later tonight.  Most were the same as the library except for the cathode voltages V4 & V5 which are in the valve chart data above.  Schematic says .035 and I have .089.  The tubes are biased to 39ma with the Hoffman bias checker.  Again I will post the other voltages on the schematic later tonight.

[EL34]

.035 volts is 35 ma of current for each power tube.

If you are showing .089 volts at the same place that I show .035 volts, then that means you are running 89 milliamps of current at idle which is way too hot.

[shortfuse]

Thats what I thought but why would it read 89ma at the resistor and 40 on the bias probes.  Actually I have it set at 37ma with the bias probes now.  I tried to turn it down tonight and just bias with the meter at the 1ohm resistor the lowest it would go was .042 and it would not amplify a signal until it was .055.  Bias probes say 37 on both meters as I have 2 hooked up at the same time and at the resistors it says .086 now.  Am I doing something wrong why would they read differently?

The rest of the voltages on the schematic are attached to this post.

[EL34]

The bias probes reads current using a meter set to ma.

On my boards you read voltage across 1 ohm resistors.

You don't need a bias probe.

You should set the power tubes to around 35ma current with a bias probe or 35 millivolts using a 1 ohm resistor.

[sluckey]

Thats what I thought but why would it read 89ma at the resistor and 40 on the bias probes.

I would check/replace that 1Ω resistor.

[shortfuse]

Both 1ohm resistors reads 1.3.  Is this enough variance to warrant replacement and cause a problem?  .3 is 30% of 1 so maybe?  Again if I turn down the bias utilizing the bias points on the board the lowest they will go is 42ma and will not produce sound it will only crackle when you hit the strings.  Bias range resistor reads 47k as per the schematic.  I would think at 89ma the tubes would red plate after playing a while would they not?  they do not seem abnormally hot or glowing hotter than normal.  It seems to me the bias probes are giving the correct readings.  As Doug stated in his last post The readings on the board and using the probes should be the same.  I will pick up 2 new 1 ohm resistors today and replace them and see what happens.
 
Only other variation I found was G2 for V44 & V55 read 8.19 but they are banded Grey, red, gold, gold so they are intended to be 8.2 which varies from the schematic of 3.9K.  Are these what is referred to as grid stoppers?  And would these have any affect on my problem?

I apologize for basic questions but I am still learning.

[EL34]

1.3 ohms = 1 ohm + .3 ohms resistance in the leads of your meter
Touch both leads together and zero them out if your meter will do that

There is something wrong with the way you are measuring.
Either your meter is funky or you are not using the correct ground reference point

Forget about the bias probe for now, use the one ohm resistors.
set your meter on DC millivolts
clip the meter leads right across one of the 1 ohm resistors
Black lead at the bottom of the resistor, red lead at the top closest to the power tube

Tell me what it says in millivolts

[shortfuse]

Well Doug the meter is not funky but the operator on the other hand????  I was using the incorrect ground point.  I was on the main ground lug and not across the resistor.  I should have remembered that from when Slucky helped me with the GA5 build.
It reads 35.7mv I adjusted to 35mv
Here is the revised data chart                       http://www.el34world.com/charts/valve/ValveData.php?e=view&f=13751
Along with the rest of the voltages attached.

[shortfuse]

Doug I have also attached a pic of the way the Hot Switch is connected to the ring of the input jack.  Should it be at the input jack like in the pic? 

[EL34]

I can't tell from the picture, it's not very sharp of focused.

The wire that comes from the switch goes to the ground lug on the input jack
The ground lug and the switch lug on the input jack are tied together
The wire coming from the switch can go to either one lug since they are both tied to ground

This is the diagram here
http://www.el34world.com/projects/hotswitch.htm

BTW, If you were not able to zero out your leads, you were actually measuring across a 1.3 ohm resistor.
Now that you know where to clip your leads, you can double check the bias with a bias probe.

Measuring the actual current is more acurate than measuring the voltage drop across a resistor
Then you can see how much diffetrence the .3 ohms makes by reading the actual current flowing

[shortfuse]

The wire that comes from the switch goes to the ground lug on the input jack
The ground lug and the switch lug on the input jack are tied together
The wire coming from the switch can go to either one lug since they are both tied to ground

Yes that is the way it is wired.  Thank you for the confirmation.
I did not do a comparison tonight on the bias but I will tomorrow to see what the difference is between the 2 methods.

Last night after I made my last post and went surfing on the forum and found this post   http://www.el34world.com/Forum/index.php?topic=5.0    so I decided to buckle my self down tonight and follow the advice.  You can not see my highlighter marks on the page as the scanner at home only does black and white.  I am glad I did this as it gave be a better understanding of the flow of the layout.  Just wish I had a better understanding of the electrical fundamentals.
As you can see by the pdf of the layout there are some discrepancies but I believe most are from the hot switch.  Some others like the grid resistors (I have read on this forum of these values being raised), bias caps lower value?
I thought I was on to something when I got a reading of 4.4k across the feedback resistor but when I replaced it the new one it read 4.4k across the resistor also and I rechecked the old one I just pulled out to find it reads 47k out of the board.  Not real sure if it should read like that or not.

But all the voltages seem in line with the schematic.  Now I have a constant noise / hum on the gain side of the switch.  Hot switch off livable Vol at 10 and MV all the way up to 8.  hot switch on, very noisy at about 7 on the volume and 4 on the MV.
Don't know if I jarred anything while taking readings tonight.  I am going to go back through 1 more time tomorrow to make sure I didn't miss or overlook something.

[EL34]

I am afraid that the more poking around and moving things that goes on will not be good.

Sounds to me like the amp worked fine before.

I was thinking that when the bias was set to 89 milliamps that the power tubes may have been plating red?

[shortfuse]

I was thinking that when the bias was set to 89 milliamps that the power tubes may have been plating red?

Doug it never really was 89ma I was checking it using the wrong ground reference and not across the resistor.  It has never been above 40 on the bias probes.
Also I did the comparison of measuring across the resistor or with the probes last night also.  There is an aprox 2ma difference, being higher checking across the resistor than with the probes. 

So I when back through the whole amp again last night to see if i missed something or measured something wrong again and had the same readings every where.
 
I will say that changing the FB resistor did help a bit with the back ground noise especially when the presence is turned up higher.  Question though why does it read 4.3k across the resistor on the board and not 47K as it is supposed to when pulled out?  And why would it make a difference installing a new one even though the old one read 47k out also?

Now here is where you will either roll your eyes or chuckle.  I started playing through it and all was great with the hot switch off.  Still very noisy with the hot switch on but not as bad as before I switched out the FB resistor.  Now where i set up and the back room to work on amps and play is kinda cramped especially when I set up the folding table to work on amps.  So if you can envision this, the head is on a table in front of me, it is plugged into the 412 behind the table and I am sitting in a chair right in front of the head on the table.  I flipped the hot switch on and started playing and it was still noisy.  My daughter asked me to look at something on the computer aprox 6 feet across the room, the chair I am using is on wheels so i slide across the room with the guitar in my lap and look at what she want me to see.  Noise is severely decreased.  I was sitting to close to the amp and it was almost the same elevation as the guitar.  Transformers and pickups were directly in line with other.  So I say to myself way to go dumbass 2 nights of wasted time and effort (not completely i did gain some knowledge following the flow of the amp and taking voltages). 
So now I proceed to check the original problem which was noise with pedals in line and the noise is still there.  I switch the input jack out and it is much better now.very quiet hot sw off and liveable hot sw on.  Why would that be if all checks out ok on the meter?

[EL34]

I don't know

None of it makes sense to me

[shortfuse]

I don't know

None of it makes sense to me

I would assume a lot of the not making sense may be the verbiage I used to post the questions and findings, in conjunction with me checking voltages incorrectly at some areas of the amp. I apologize for misconceptions and my inexperience.


1.  Here are the specifics on this thread.  Before this thread of posts;
•   The amp was almost always dead quiet hot switch off.  And had no more background noise than any of my older Marshall or Fender amps with the hot switch on.  It always accepted pedals with no issues except for a few older ones that do not have true bypass which I do not use with this amp (I basically only use a chorus, reverb and delay with this head).  I usually do not use any gain pedals with the amp but if I do chain in a distortion pedal it is never with the hot switch on, so they were basically passive hot switch on.
•   About a month ago it started to not accepting pedals at all.  It didn’t matter what type tuner, modulation, distortion, ect… and would create a constant background noise as soon as the stand by switch was switched on, and a pedal was in line to the amp hot switch on or off.  But the amp was fine with just using the guitar and no pedals.
•   When I started this post of course I had all kinds of questions about cords, pedals, outlets in the house, ect…  I tested everything and it was none of the above as I use 6 different amps in this room with the same cords, pedals, ect…
•   I switched pre amp & power tubes with a known good set from another working amp in the same arrangement as the amp they came out of in the 2034 and vise versa put the pre amp & power tubes from the 2034 in the Marshall and it worked just fine.  So I concluded it was not the pre amp or power tubes.
•   In my inexperience I decided to lift the board to check voltages because the layout in the library did not match this amp.  In the process of doing this 2 wires became detached from the board (wiper of the master and ground of the volume).  I reattached these and the problem went away????  Cant explain why.
•   3 uses later the same thing happens again it will not accept pedals but is fine with just the guitar alone.  Then I started checking everything.  I also did trace out everything and found the discrepancies between what I have and the schematic.
•   Then again in my inexperience I was sitting to close to the amp on the bench with the guitar after I replaced the NFB resistor, and posted that the problem became worse causing more background noise & hum.  This was figured out by accident as posted above.  The post prior to finding out I was too close to the amp was most likely miss leading.
2.  Components replaced;
•   FB resistor because it measured 4.2k across it and the schematic and banding on the resistor called for 47k.  Only to find out that the new one reads the same installed on the board and the old one reads 47k out of the amp.  As crazy as it sounds the change of the resistor did help a little bit in my opinion.  Especially with the presence turned way up.  
•   The input jack was replaced after I figured out I was too close to the amp as I purchased a new one and it was a cheap easy item to replace.  And because it was in the input circuit.  It also had a small impact on decreasing the background noise, again in my opinion.
•   I did replace the tubes with a new set I was saving for the next build because they were the original tubes from the early to mid 90’s and I have no idea how many hours are on this amp.
•   The amp is quieter now, but not as quiet as it was prior to the 1st instance of this happening.  Which is the frustrating part because I know what it could be like?
3.  Specific questions
•   Can someone please explain the negative feed back resistor to me?  Should it read like that or should it read 47k across the resistor on the board?  Or could this be a potential problem also?
•   Should the mv reading across the 1 ohm resistor be the same reading as you would get with the bias probes if I had a meter I could zero out?
•   I know this is suggestive but could I have a bad Cap, or one that is going bad intermittently right now that will fail later?  They are the original ones installed back in the early to mid 90’s.  I had a problem with a re-issue twin once that caused some crazy stuff to happen that I could not find until the cap finally failed.
•   Could the hot switch check out ok on the meter but have an issue internally causing this type of symptom?  

Any other suggestions would be greatly appreciated.

Again I apologize for my inexperience with the technical side of amp building and repair which leads to miss guided questions.  Some day I will be able to contribute to this board rather than being a sponge.

Steve

[sluckey]

Can someone please explain the negative feed back resistor to me?  Should it read like that or should it read 47k across the resistor on the board?  Or could this be a potential problem also?

You can't always read resistance values in a circuit because there may be something in parallel that will skew the reading. Look at the drawing below. I've eliminated all the circuitry that has no bearing to your bogus reading. Imagine your meter leads across the 47K resistor. But, look at the rest of the picture. Follow the circuit down from the bottom of the 47K resistor and you pass thru a 5K pot to ground. Now jump over to the ground connection on the OT secondary and follow up thru the secondary (probably only 1Ω or so) and continue up and to the left and you end up at the top of that 47K resistor. In other words, your meter is across that 47K for sure, but it's also across the 5K pot. If you do the math you'll see that the total resistance of that 47K || 5K is about 4.5K. So the meter reading is really accurate. It's the same as if you took a 47K and a 5K and twisted the leads together in parallel. The total resistance will always be less than the smaller resistor.

Should the mv reading across the 100ohm resistor be the same reading as you would get with the bias probes if I had a meter I could zero out?

I'm assuming you meant 1Ω resistor, right? I thought you finally got the readings to agree within reason? Any small difference would be due to non perfect 1Ω resistors and/or the accuracy of your meter, especially since you would be measuring volts in one case and current in another case. I would not worry about that. But a huge difference like you first described could mean a faulty meter, bias probes, resistor, or procedure error.

[shortfuse]

Thank You Steve, and yes that should have been 1ohm resistor not 100. 
I am going to just play this one a little more often now and see if it stays the same or gets worse again.
And thank you Doug.